The thermal cracking of hydrocarbons, such as gaseous paraffins, up to naphtha and gas oils to produce lighter products, particularly lighter olefins is commercially important. A leading commercial process for thermally cracking such hydrocarbons to olefinic products is steam cracking wherein the hydrocarbons are pyrolyzed in the presence of steam in tubular metal tubes or coils (pyrolysis tubes) within furnaces. Studies indicate that substantial yield improvement results as temperature is increased and reaction time, as measured in milliseconds, is decreased.
Conventional steam cracking is a single phase process wherein a hydrocarbon/steam mixture passes through tubes in a furnace. Steam acts as a diluent and the hydrocarbon is cracked to produce olefins, diolefins, and other by-products. In conventional steam cracking reactors, feed conversion is typically limited by the inability to provide additional sensible heat and the heat of cracking in a sufficiently short residence time without exceeding allowable tube metal temperature limitations. Long residence times at relatively high temperatures are normally undesirable due to secondary reactions which degrade product quality. Another problem which arises is coking of the pyrolysis tubes. The thickness of coke on the inside walls of the metal surfaces that come into contact with the feedstock to be cracked progressively increases. This coke layer is objectionable from the point of view of loss of heat transfer which leads to high tube metal temperatures. Steam cracking processes are described in U.S. Pat. Nos. 3,365,387 and 4,061,562 and in an article entitled "Ethylene" in Chemical Week, Nov. 13, 1965, pp. 69-81, all of which are incorporated herein by reference.
The typical feedstocks to a steam cracking process unit, for the purpose of making olefins, are relatively expensive feedstocks such as ethane, liquefied petroleum gas, naphtha, and gas oils. It would be a significant economical advantage to be able to produce olefins from heavier feedstocks, such as residual feeds, which are substantially cheaper than gas oils. Residual feeds typically contain substantial amounts of asphaltene molecules which are usually responsible for a significant amount of the Conradson carbon residue and metal components in the feed. They also contain relatively high levels of heteroatoms, such as sulfur and nitrogen. Such feeds have little commercial value, primarily because they cannot be used as a fuel oil owing to ever stricter environmental regulations. They also have little value as feedstocks for refinery processes, such as fluid catalytic cracking, because they produce excessive amounts of gas and coke. Also, their high metals content leads to catalyst deactivation. They are generally unsuitable for use in steam cracking process units because of excessive cracking, coke formation in the pyrolysis tubes leading to overheating and equipment plugging. Thus, there is a need in petroleum refining for greater utilization of such feedstocks, or to upgrade them to more valuable cleaner and lighter feeds.
An attempt to overcome these problems was made in U.S. Pat. No. 2,768,127 which teaches contacting the residua feedstock in a fluidized bed of coke particles maintained at a temperature from about 675.degree. C. to 760.degree. C. While such attempts have been made to overcome these problems, there remains a need for improved processes having better control of solids and vapor residence times.